Downhole technology, such as systems and methods for drilling oil wells and other subterranean holes or shafts, has historically relied on mechanical gearboxes for regulating output shaft speed. These mechanical gearboxes typically have a fixed gear ratio, which results in a fixed operating envelope whereby the drilling system is designed to produce a maximum torque. Typical gearboxes can thus deliver a maximum torque as needed, but this conversely results in slower overall drive shaft angular speed due to elevated gearbox ratios that may be required. With fixed gear ratios, either angular speed or torque may need to be sacrificed. This is particularly problematic in attempting to maintain peak power of an electric driving motor. Ideally, the torque and angular speed would respond such that the peak power and premium performance of the motor is maintained. In a peak power phase, the drive motor is operating at its highest possible efficiency.
Continuous Variable Transmissions (CVT) have been known for a long time and refer to the general class of gearbox transmissions that can automatically and continuously adjust between a minimum and a maximum gear ratio. A variety of CVTs have been developed and utilized in various industries, particularly the automobile industry in order to optimize engine performance and improve fuel economy. CVTs have the benefit of allowing the input shaft to maintain a constant angular velocity over a range of output velocities. Several types of CVTs include: hydrostatic, toroidal, variable-diameter pulley, magnetic, infinitely variable, ratcheting, nautical incremental, cone, radial roller, and planetary transmission systems.
In the context of downhole applications, some conceptualizations of CVT's have been disclosed, however. For instance, U.S. Pat. No. 7,481,281 to Schuaf, the entirety of which is incorporated herein by reference, generally discloses a hollow disc toroidal CVT in FIG. 16 and a ball toroidal CVT in FIG. 17 of Schuaf. These toroidal CVT's may be utilized in connection with a hydraulic, or fluidic, turbine assembly, illustrated in FIG. 21 of Schuaf. In view of Schuaf, a problem remains with downhole applications in that designed CVTs are still too large for some downhole applications, are limited in their ability to accommodate extremely high or sudden rotation resistance differences between the input and output shafts, as may be experienced from material resistance in downhole applications, and are limited to usage with hydraulic drive motors. As such, a more efficient and adaptable transmission system is needed. Moreover, Schuaf does not allow for automatic transmission ratio adjustment as a function of output torque.